Flow and dynamo measurements in the HIST double pulsing CHI experiment

Transcription

1 Innovative Confinement Concepts (ICC) & US-Japan Compact Torus (CT) Plasma Workshop August 16-19, 211, Seattle, Washington HIST Flow and dynamo measurements in the HIST double pulsing CHI experiment M. Nagata, T. Higashi, M. Ishihara, T. Hanao, K. Ito, K. Matsumoto, Y. Kikuchi, N. Fukumoto, T. Kanki 1) University of Hyogo, Japan Coast Guard Academy 1) Outline 1) Introduction 2) HIST device and diagnostics 3) Experimental topics a) Flows and density profile b) Two-fluid dynamo measurements c) Propagation of magnetic fluctuation 4) Summary

2 Introduction Coaxial Helicity injection (CHI) is an efficient current-drive and start-up method which was used in many spheromak and ST experiments. A critical issue for CHI is achieving a good energy confinement A new approach of CHI ; Refluxing or Multi-pulsing CHI The multi-pulsing scenario of CHI aims to achieve simultaneously a quasi-steady sustainment and good confinement. ExB rotation Driven Phase 3D MHD simulation Decay Phase Low q ST B t Flow n=1 kink mode time Self organizing

3 Multi-pulsing CHl for ST configurations The multi-pulsing CHI (M-CHI) discharges on SSPX at LLNL were successfully demonstrated for a high temperature spheromak. (see Ref.[1,2]) Application of the M-CHI to ST configurations What mechanism of current drive is different from spheromak? Flow, Dynamo, Mode structure, etc. High-q ST Low-q ST Spheromak Central open flux column plays an important role in driving a current. A purpose of this experiment is to explore characteristics of the M-CHI driven ST. [1] S. Woodruff, et al. PRL 9, (24) [2] E.B. Hooper, PPCF 53, 858 (211).

4 HIST device and double-pulsing CHI HIST R=.3 m, a=.24 m, A=1.25 n e =.5-1 x 1 2 m -3 T e, T i = 1-4 ev I t < 15 ka S*=R/l i ~1 l i =(c/ω pi )=2~3 cm TF coil current Spheromak, Low q ST: q ~ I tf (= 3 ka) / I t <1 High q ST: q ~ I tf (= ~ 15 ka) / I t >1 Power supply system for double-pulse Formation capacitor banks V = 3-1 kv, C =.6 mf Injection current : I g ~ 3-6 ka Sustainment capacitor banks First pulse : V < 9 V, C = 336 mf Second pulse : V < 9 V, C = 195 mf Injection voltage: V g ~ 4 V Injection current: I g ~ 15 2 ka

5 Dynamo-Mach Probe Measurement 3-axis flows and 3-axis magnetic fields are simultaneously measured. - Mach probe analysis - Hutchinson model : unmagnetized Ion Mach Number M i : C s =3 km/s (T e =T i ) J up upstream rod current J down downstream rod current V i : ion flow, C s : ion sound velocity, M i : ion Mach number, M c : proportionality constant, T e (T i ): electron (ion) temperature,γ e (γ i ): specific heat ratio for electron (ion), r p : probe radius, ρ i : Larmor radius (~ 1 cm)

6 Double pulsing CHI discharge Single pulsed discharge 1 st pulse 2 nd pulse Double pulsed discharge Plasma current Averaged density Inner edge Bp Outer edge Bp λ in OFC λ in Core OII Doppler ion temp. By secondly pulsing the MCPG at t = 1.5 or 2.5 ms during the partially decay phase, total plasma current is effectively amplified against the resistive decay. The core current density is generated due to dynamo. The sustainment time has increased up to 6-8 ms which is longer than that in the single CHI case. The edge λ in the OFC is larger than the core λ, causing helicity transport. Doppler ion temperature increases after the second gun pulse.

7 Flux amplification Second gun pulse Time (ms) High-q ST Poloidal Flux >Bias flux Ψp > Ψ p.bias Bias flux Bp in OFC region Bp.in in closed flux region Flux Amplification A Ψ > 4 ~ 5 # Note that A Ψ includes the OFC Bp.out in closed flux region Injection gun current B p in the closed flux region increases with an increase in the gun current

8 Internal magnetic field profiles 1 st pulse 2 nd pulse B p (R) M-CHI process Driven phase Decay phase OFC Toroidal current Magnetic field density J t B p and B t Magnetic axis Separatrix Time (ms) OFC Bp is enhanced in the OFC. Magnetic axis moves outwardly. J t becomes from a hollow to a peaked profile. t=2 ms t=6 ms t=7 ms

9 Diamagnetic properties of Open Flux Column The toroidal field B t in the OFC is decreased from the vacuum field, i.e. diamagnetic due to a high pressure gradient p. OFC radius ~ R diamag Separatrix position R diamag.12~.18 m

10 Flows and density profiles Poloidal flow v p = Er B 2 B t p B i t 2 enb Poloidal shear flow Diamagnetic current I tf OFC j diamag Toroidal flow Diamagnetic properties of OFC P Radial electric field E r ion E r ỊI t Density ( I sat ) negative edge E r electron Steep gradient OFC E r en i p Z i i ( v B p t v t B p )

11 Two fluid dynamo effect Generalized Ohm s s law ~ ~ ~ η j E = v B ) + < v~ B > ( j B ) /en < j B > ηj E η j = E + v B j B ( Hall dynamo /en+ ~ ~ ~ ~ ~ =< v ~ B > < j B > /en < v B > MHD dynamo ~ ~ ~ E v j /en ~ B e B ve = ExB drift # Diamagnetic dynamo term does not appear explicitly in the parallel meanfield Ohm s law ~ ~ < pe B > Diamagnetic dynamo en B * Private communication with Dr. K. McCollam B p~ enb Diamagnetic drift e p e /en Electron dynamics e je j 2 i 2 B B /en # Diamagnetic current j diamag due to electron and ion diamagnetic drift contributes on ~ B ~ Hall dynamo term p ~ B ~ p i

12 Hall and MHD dynamo measurement Measurement of three components of fluctuating velocity, current density and magnetic field at a radial position r θ z Hall dynamo probe (5x5x5 mm) Incorporating Rogowski loop and flux loop Hall dynamo probe MHD dynamo probe E E t t =< δv δb =< δj δb > > t t =< δv =< δj z z δb δb r r δv δj r r δb δb z z > > out of phase in phase

13 Dynamo balances Ohm s s law MHD Hall Toroidal current density MHD 1 st st pulse 2 nd nd pulse Core Core Core OFC ηj ~ ~ ~ ~ E = < v B > < j B > η spitzer Parallel mean-field Ohm's law = MHD dynamo Ψ p ( r) E ( r) = V /L g t 2 π r η OFC region = η Z eff Hall dynamo (kt ) 3 / 2 2 ~ Core region e ln /en I sat Λ Ωm Hall Toroidal current density OFC OFC V E ηj g 1 ~ 2 1 ~ 3 ~ V V/m V/m ~ ~ ~ < v~ B > < j > /en V/m B V g E ηj 2 V V/m 4 ~ 12 V/m 6~ 14V/m

14 Radial profile of dynamo electric field ~ ~ < v B > OFC ~ ~ < j B Core > /en Hall dynamo Collisional drag of electrons on the ions accelerates the ions in the core. Electrons become slow down due to anti-dynamo E MHD MHD dynamo Anti-MHD dynamo Ions are accelerated due to E Hall in the direction of the current in the OFC. Hall dynamo driven current in the OFC is the same direction as the mean current. MHD anti-dynamo electric field in the OFC reduces the mean current. Electron locking model [1] δω ce τ ie >δ/r [1] T.R. Jarboe et al., Nucl. Fusion (211).

15 Radial propagation of magnetic fluctuation Gun OFC region Closed flux region λ gun > λ OFC > λ core Helicity flow 2nd-pulse f (khz) #1416 R (mm) OFC Second-gun-pulse 8 khz 2-4 khz ~ B ( t ) #1416 Red R=.25m Black R=.1m R=.75 m R=.175 m R=.25 m R=.3 m 2-4 khz 8 khz Time (ms) Time (ms)

16 Axial propagation of magnetic fluctuation ~ B ( t ) Propagation by Alfven speed Z (m) midplane Second-gun-pulse #1416 ~ B ( t ) ~8 khz Red Z=.784m Black Z=.216 m Time (ms) Z=.926 (m) Z=.784 (m) Z=.642 (m) Z=.5 (m) Gun Second-gun-pulse f (khz) 8 khz Time (ms) #1416

17 Summary The HIST device has been developed towards high-β and quasi-steady-state sustainment of high-q and low-q ST plasmas by Multi-pulsing CHI method. We have successfully demonstrated the flux/current amplification and sustainment of the plasmas in the double gun pulse experiment. We have observed the poloidal flow shear between the OFC region and the closed flux region. The ion diamagnetic drift due to a steep density gradient observed there could account for the flow reversal. We have measured simultaneously the Hall and MHD dynamo spatial profile. The relative contributions of the different dynamo electric field on the driven current have been investigated to verify mean Ohm s law balance. The Hall dynamo acts to generate the mean current density in the both regions, although anti-mhd dynamo is effective in the OFC region. The observed spatial propagation of magnetic fluctuations suggests that helicity is transported to the core from the OFC connected with the gun electrodes during the driven-phase in the multi-pulsing CHI.

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